The present invention relates to a hollow, reduced weight poppet valve in which a cap is integrated by welding to a flare-shaped fillet area formed at one end of a valve stem and a method for manufacturing the same.
Japanese Laid-Open Patent Publication No. 299816 of 1994 discloses, as shown in FIG. 14 and FIG. 15, a valve having a fillet area 4 which is opened like a flare formed at the end portion of a cylindrical stem portion 2. A cap 6 is integrated into the fillet area 4 by welding, therefore forming a hollow poppet valve wherein numeral 7 indicates a welded portion.
Also, as shown in FIG. 14, a face 8 is secured at the cap 6 while a face 8 is provided at the fillet area 4 in the valve shown in FIG. 15.
However, it has been found, through an examination of the Vickers hardness hereinafter called xe2x80x9cHVxe2x80x9d in the vicinity of the face on such a prior art poppet valve that, although the face needs a hardness of at least 200 HV to avoid premature wear, that no such hardness was attained.
FIG. 16 and FIG. 17 show the hardness distribution on a longitudinally sectional plane of the prior art valve in the vicinity of the cap-welded portion shown in FIG. 14 and FIG. 15. Although areas c, d, e, f and g other than the welded portion xe2x80x9caxe2x80x9d and an area b surrounding the area xe2x80x9caxe2x80x9d have a hardness that is more than 200 HV required for the valve, the hardness of the welding area xe2x80x9caxe2x80x9d was 100 through 149 HV, and of the area facing the welded portion xe2x80x9caxe2x80x9d (that is, the area surrounding the welded portion xe2x80x9caxe2x80x9d) was 150 through 199 HV. In either structure, the hardness of the area that is roughly the half of the face 8 was less than 200 HV which is insufficient for the face. In addition, in FIG. 16, the areas c through g having 200 HV or more are such that, from a position nearer to the welded portion xe2x80x9caxe2x80x9d, the area c has a hardness of 200 through 249 HV, area d has 250 through 299 HV, area e has 300 through 349 HV, area f has 350 through 399 HV, and area g has 400 HV or more.
The present inventor has examined causes as to why the hardness of the face 8 was lowered to below 200 HV and has found that, since the cap 6 is welded in the vicinity of the face 8, and the face 8 is influenced by the welding temperature, it was difficult for the face 8 to keep the necessary hardness. Further, in the structure shown in FIG. 14, the inventor has observed that a load (stress) caused by valve seating is concentrated in the vicinity of the welded portion 7 such that the welded portion 7 may be easily fractured. And, in the structure shown in FIG. 15, there is a fear that stress is concentrated onto the bent portion 5 in the vicinity of the welded portion 7, whereby the welded portion 7 may be broken.
The present invention was developed to solve the above-mentioned problems. It is therefore an object of the invention to provide a hollow poppet valve whose property of high strength to weight is excellent by designing it such that no metal softening influence due to welding is provided onto the face and no stress is concentrated at the welding portion.
In order to achieve the object, a hollow poppet valve having a fillet area opened like a flare, which is formed at one end of a cylindrical stem portion, and a cap integrated by welding at the opened edge portion of the fillet area, is provided such that the opened edge portion of the fillet area is formed to be thicker than the other areas, and a face is formed at the thicker portion.
The open edge portion at which a cap is welded is thicker than the other areas to reduce the influence of the heat of welding on the face.
By making the open edge portion of the fillet area thicker, adverse metallurgical influences due to welding heat on the face can be evaded, whereby the face can be maintained at a desired hardness. By making the other areas of the valve which are not significantly influenced by the heat of welding thinner, the total weight of the valve can be reduced.
Also, since the face which is brought into contact with the seat face at the cylinder side is formed at the fillet area (the outer circumferential side), the concentration of load (stress) at the welded portion secured inside of the thicker portion is avoided.
Also, a hollow poppet valve is featured in that the welded portion of the cap is spaced by at least 0.5 mm or more, and more preferably 0.8 mm or more from the face.
Such spacing makes it possible to evade the unwanted metallurgical influence of welding heat onto the face.
Also, a hollow poppet valve is featured in that the cap is welded by an electron beam or laser beam.
In the electron beam welding or laser beam welding, since the beam diameter is small, the width of the welded portion is narrowed, and the welding can be instantaneously carried out at a high temperature. Accordingly, unwanted thermal influences on the face can be further reduced.
Further, a hollow poppet valve is featured in that the thicker portion is bent at the open edge portion of the fillet area so that the longitudinal section thereof becomes a laterally turned V, and is formed so as to extend therefrom, wherein the inside of the bent portion includes an annular stage portion for receiving the edge of the cap so that the cap is solely supported by the stage portion and weld joint.
Since stress is concentrated at the bent portion, no stress is accordingly concentrated at the welded portion spaced away from the bent portion. Also, due to the shape of the inside of the bent portion, the stress concentrated at the bent portion can be dispersed to the entire bent portion.
Further, a hollow poppet valve is featured in that the inner circumferential face of the thicker portion at the fillet area is constructed of a cylindrical face centering around the axial center of the stem portion, and the face is constructed of a tapered plane inclined in a range from 25 through 45 degrees with respect to the axial center of the stem portion.
With such a construction, it is easy to form the thicker portion and to space the welded portion away from the face.
A hollow poppet valve is feature in that the aforementioned stage portion for engagement of the cap positions the cap with respect to the fillet area. That is, the stage portion can function as a positioning member when welding the cap to the fillet area.
Also, the cap engagement stage portion functions so as to space the welding portion away from the stress concentrated part in the fillet area.
Still further, the cap engagement stage portion carries the cap from its downside, and can function so as to relieve the load acting on the welded portion resulting from pressure operating on the fire-contacted side of the cap.
Also, a method of manufacturing a hollow poppet valve comprises the steps of press-forming a cup-shaped material in a cold state, drawing and forming so as to draw, in a cold state, a cylindrical stem portion having a fillet area opened like a flare at one end thereof and welding a cap on the open edge portion of the fillet area, wherein the press-forming step forms a flange-like thicker portion at the open edge portion of the cup-shaped molded member, and the drawing and forming step forms a thicker portion, on which a tapered plane is formed, at the open edge portion of the fillet area of the molded member.
It is preferable that an electronic beam and a laser beam is used to weld the cap, and that the drawing step provides a cap engagement stage portion formed at a position spaced by at least 0.5 mm or, more preferably 0.8 mm or more from the face inside the open edge portion of the fillet area.